Δημοφιλή αποσπάσματα

Σελίδα 281 - ... crystallisation ; and though, from the influences of other mechanical forces to which the rocks have been exposed, and from the varying proportions of their constituent ingredients, we cannot expect the angles to present the exact definition which a crystal of the pure mineral would have, still there is every appearance of the plane-lines of shrinkage and jointing having been guided in many cases, if not in all, by planes of crystalline cleavage, in consequence of these being those of least resistance.

Σελίδα 406 - With all the harder rocks the heat produced in the metal surroundings by the complete crushing was easily perceptible by the hand, and was so great with some of the granites and porphyries as to necessitate a delay for the apparatus to cool. Both Mr. Mallet and Prof. Rankine were of opinion that in the crushing of a rigid material such as rock almost the entire mechanical work (with the exception of a small residue of external work) reappears as heat.

Σελίδα 211 - the rise of lava in a volcanic vent is occasioned by the expansion of volumes of high-pressure steam, generated in a mass of liquefied and heated matter within or beneath the eruptive orifice.

Σελίδα 83 - ... increased, its ability to transport material increased not by an arithmetical ratio but by a geometrical one (p. 226). A curious consequence results from this law, when we estimate the force of the current by weight of the largest block of a given form which it is capable of transporting. Thus estimated, the force varies as the sixth power of the velocity of the current. (Hopkins, 1848, p. 92) Or put more exactly : Therefore the moving force of a current estimated by the volume or weight of the...

Σελίδα 138 - Dr. Livingstone found in Africa (12° south latitude, 34° east longitude) that surfaces of rock which during the day were heated up to 137° Fahr. cooled so rapidly by radiation at night that, unable to bear the strain of contraction, they split and threw off sharp angular fragments from a few ounces to 100 or 200 pounds in weight. In the plateau region of North America, though the climate is too dry to afford much scope for the operation of frost, this daily vicissitude of temperature produces...

Σελίδα 437 - The former bottom of the ocean will then (after the lapse of ages, and when a fresh state of equilibrium is attained) acquire a temperature corresponding to its then actual depth ; while a point as deep below it, as itself is below the surface, will have acquired a much higher temperature, and may become actually melted, and this without any bodily transfer of matter in a liquid state from below.

Σελίδα 83 - The least velocity, or that of the particles in contact with the bed, is about as much less than the mean velocity as the greatest velocity is greater than the mean. In ordinary cases, the least, mean, and greatest velocities may be taken as bearing to each other nearly the proportions of 3, 4, and 5.

Σελίδα 206 - It plunged into the sea with loud detonations. The burning lava, on meeting the waters, was shivered like melted glass into millions of particles, which were thrown up in clouds that darkened the sky and fell like a storm of hail over the surrounding country.

Σελίδα 242 - ... areas rising, with volcanic matter every now and then bursting forth through the vents or fissures with which they are traversed. We see other wide spaces slowly sinking without any volcanic outbursts ; and we may feel sure, that this sinking must have been immense in amount as well as in area, thus to have buried over the broad face of the ocean every one of those mountains, above which atolls now stand like monuments, marking the place of their former existence.